'Sloppy' sperm mixing and intraspecific variation in sperm precedence (P-2) patterns

Sperm precedence patterns are typically highly variable within land between ) species. Intraspecific variation in sperm precedence (measured as P-2, th e proportion of progeny fathered by the last male Co male) is frequently se en as a candidate for adaptive interpretation through either male effects ( e.g. body size), female effects (e.g. cryptic female choice) or an interact ion between the two. Here we show, using computer simulation, that if ejacu lates divide into a number of 'packets' and packets from two males mix rand omly then a variety of patterns of sperm precedence may result. We term thi s process 'sloppy' mixing. if ejaculates break into a small number of packe ts, bimodal P-2 distributions are predicted. As the number of packets is in creased, then a complex series of changes through multimodal and flat to un imodal distributions results. Sloppy mixing can thus result in many of the observed P-2 distributions. Sloppy mixing is unlikely to change the predict ions of adaptive models of sperm competition.